Performing active interrogation of battery packs in situ to obtain precise SOC and SOH estimates

ABSTRACT

A characteristic, such as State Of Health (SOH) or State Of Charge (SOC), is estimated for an Energy Storage System (ESS) by supplying a pre-determined signal to the ESS, measuring a response signal output by the ESS, and obtaining an impedance spectrum of the ESS. In one example, the ESS is one of several electrochemical battery packs of an electric vehicle. The pre-determined signal is a current signal generated by a switching power converter that transfers charge from the battery pack to other battery packs or transfers charge from the other battery packs onto the battery pack. The pre-determined signal is generated without disrupting any load supplied by the battery packs. The battery pack outputs a voltage signal in response to receiving the pre-determined current signal. A processor obtains an impedance spectrum using the current and voltage signals, and thereby obtains an SOH and SOC estimate of the battery.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of, and claims the benefit under 35U.S.C. § 120 from, nonprovisional U.S. patent application Ser. No.13/868,075, entitled “Performing Active Interrogation Of Battery PacksIn Situ To Obtain Precise SoC And SoH Estimates,” filed Apr. 22, 2013.U.S. patent application Ser. No. 13/868,075 claims the benefit under 35U.S.C. § 119 of U.S. provisional application Ser. No. 61/635,988,entitled “System And Method For Determining Battery Pack Health InSitu,” filed Apr. 20, 2012. The entire subject matter of theaforementioned patent documents is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates generally to Battery Management Systems(BMSs), and more particularly to estimating State of Charge (SOC) andState of Health (SOH) of a battery pack.

BACKGROUND INFORMATION

Clean energy systems such as electric vehicles and electrical powergrids require energy storage mechanisms. An Energy Storage System (ESS)is typically used in such a system to store energy. In order to operateefficiently, the system requires estimations of certain characteristicsof the ESS. Of particular interest are State Of Health (SOH) and StateOf Charge (SOC). The SOH is a characteristic of the ESS that indicatesthe state of degradation of the ESS. The SOC is a characteristic of theESS that indicates the amount of charge or energy remaining in the ESS.Although methods exist for determining the SOH and SOC characteristicsof single battery cells, these techniques are often ineffective indetermining the SOH and SOC characteristics of battery packs containinghundreds of cells. A more effective technique for determining the SOHand SOC characteristics of battery packs is desired.

SUMMARY

A characteristic, such as State Of Health (SOH) or State Of Charge(SOC), is estimated for an Energy Storage System (ESS). The ESS is partof a plurality of ESSs. In one embodiment, the ESS is an electrochemicalbattery pack (referred to as “battery pack”), and the plurality ofbattery packs are part of a powertrain of an electric vehicle. Each ofthe battery packs includes a plurality of electrochemical cells and aBattery Management System (BMS). Each battery pack is coupled to abattery controller that includes a switching power converter, a voltagesensing circuit, and a processor. The power train also includes a motorand a motor controller. Each of the battery pack controllers and themotor controller are coupled to a power bus and a communication bus. Thepower bus is used to transfer charge between the components. A PowerControl Unit (PCU) controls the overall powertrain system bycommunicating with each of the controllers via the communication bus.

The SOH is a characteristic of the battery pack that indicates the stateof degradation of the battery pack. During the lifetime of the batterypack, the battery pack undergoes charge and discharge cycles. Suchcycles cause the battery pack to degrade because of the physical andchemical variations that occur while the battery pack is operating. TheSOH is usually represented as a percentage value corresponding to: thebattery pack's total capacity at a given time, the number of charge anddischarge cycles remaining, or the likelihood the battery pack will failwithin a given time. A SOH of 100% would indicate a new battery, whereasa SOH of 0% would indicate the battery is inoperable. The SOHcharacteristic is useful to determine when a battery pack should bereplaced with a newer battery pack to improve efficiency and overallrange of the electric vehicle.

The SOC is a characteristic of the battery pack that indicates theamount of charge remaining in the battery pack. The SOC is typicallyrepresented as a percentage value. A SOC characteristic of 100% wouldindicate the battery pack is fully charged, whereas a SOC of 0% wouldindicate the battery pack no longer has any usable charge. The SOCcharacteristic is valuable to estimate the amount of range left in theelectric vehicle.

During operation, the PCU determines that the SOH characteristic is tobe estimated for one of the battery packs. This is also referred to as“testing” the battery pack. The SOH characteristic of the battery packis estimated by generating a pre-determined signal using the the batterypack and the switching power converter. The pre-determined signal may bea constant current signal, an impulse current signal, a sinusoidalcurrent signal, a step current signal, or a triangular current signal.The pre-determined signal is generated as a result of the switchingpower converter supplying current to the battery pack (referred to as“charging”) or the battery pack supplying current to the switching powerconverter (referred to as “discharging”). A single pre-determinedcurrent signal may be generated, or a plurality of pre-determinedcurrent signals may be generated.

The pre-determined current signal is generated by sinking current fromthe battery pack being tested onto the other battery packs, sourcingcurrent from the other battery packs onto the battery pack being tested,or sinking and sourcing current between the battery pack being testedand the other battery packs. Charge transfers between the battery packsthrough the switching power converters within the battery packcontrollers. Whether the pre-determined current signal is supplied bycharging or discharging the battery pack, the electrical load or sourcecontinues to receive or supply power from or to the DC power bus andmaintain complete functionality during the time interval that the SOH isbeing estimated. Other battery packs not under test are controlled toadjust their power delivery to the load or power receipt from the sourceto compensate for the battery pack under test. In another example, aseparate hardware device is used to generate the pre-determined signal,such a Silicon Controlled Rectifier (SCR), or a power transistor and acapacitor.

In response to receiving the pre-determined current signal, the batterypack outputs a response signal in the form of a voltage signal. Thevoltage signal is measured by the voltage sensing circuitry within thebattery pack controller that detects the voltage present between twoterminals of the battery pack. The measured voltage signal represents atime-domain response signal. In another embodiment, the voltage signalis measured by the BMS.

Next, the processor performs signal processing operations on themeasured voltage signal and the pre-determined current signal todetermine an estimation of an impedance spectrum of the battery pack.The signal processing involves Fourier transform operations, such as aDiscrete Fast Fourier Transform (DFFT). Standard Spectral DensityEstimation techniques may be preferred if the presence of noise issignificant. The result of such signal processing operations is anestimation of the impedance spectrum of the battery pack. The impedancespectrum estimation provides resistance (real component of impedance)and reactance (imaginary component of impedance) information of thebattery pack across various frequencies.

Next, the processor uses the resistance and reactance informationobtained from the impedance spectrum estimate of the battery pack todetermine a value indicative of the SOH or SOC characteristic of thebattery pack. In one embodiment, the processor uses a pre-determinedequation to process the current and voltage information. Thepre-determined equation may include weighted values that are empiricallydetermined through lab testing of a similar battery pack. In anotherembodiment, the processor performs a Complex Nonlinear Least Squares(CNLS) operation to the impedance spectrum estimate information todetermine parameter values for an equivalent circuit model of thebattery pack. In yet another embodiment, the processor performs a directcomparison of regions of the impedance spectrum estimate of the batterypack to determine the value indicative of SOH and predict failures inthe battery pack. In yet another embodiment, the processor uses one ofthe above methods to estimate SOC and uses one of the above methods toestimate SOH, thus determining SOC and SOH from a single set ofimpedance spectrum estimation data.

In accordance with another novel aspect, the processor may estimate theSOC characteristic or the SOH characteristic by estimating the internalresistance of the battery pack. A pre-determined constant current signalis generated using the battery pack to be tested and the switching powerconverter. The constant current signal is generated as a result of usingthe switching power converter to transfer charge between the batterypack and the power bus. Next, a voltage between two terminals of thebattery pack is measured using the voltage sensing circuit. Next, thedifference between the measured voltage and the Open Circuit Voltage(OCV) is calculated to obtain the change in voltage. Next, the internalresistance is calculated by dividing the change in voltage by theconstant value of the constant current signal. Next, the processorperforms a look-up table operation using the determined internalresistance. If the battery pack is of a type that exhibits a correlationbetween internal resistance and the SOH characteristic and theestimation of the SOC characteristic is known, then the processorperforms a look-up operation on a SOH look-up table. If, on the otherhand, the battery pack is of a type that exhibits a correlation betweeninternal resistance and the SOC characteristic, and the estimation ofthe SOH characteristic is known or the internal resistance changesminimally with the SOH characteristic, then the processor performs alook-up operation on a SOC look-up table. The SOH table and the SOCtable are also determined empirically for each of these differentbattery types.

In another embodiment, the ESS is an electrochemical battery pack, andthe plurality of electrochemical battery packs are part of an electricalpower grid. A controller utilizes the methods described above todetermine the SOH characteristic and the SOC characteristic of anyindividual battery pack. The SOH and SOC characteristics are determinedwithout disrupting any electrical load or source that sources or sinkscurrent from or to a power line of the power grid that is connected tothe plurality of electrochemical battery packs.

The foregoing is a summary and thus contains, by necessity,simplifications, generalizations and omissions of detail; consequentlyit is appreciated that the summary is illustrative only. Still othermethods, and structures and details are set forth in the detaileddescription below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, where like numerals indicate like components,illustrate embodiments of the invention.

FIG. 1 is a perspective diagram of a portion of an electric vehicle 100.

FIG. 2 is a simplified conceptual diagram of the electric powertrain 115of the shuttle bus 100.

FIG. 3 is a more detailed diagram of battery pack 101, batterycontroller 116 and PCU 120.

FIG. 4 is a diagram showing how an impedance spectrum for anelectrochemical battery shifts over a batteries lifetime.

FIG. 5 is a plot 160 of internal resistance as a function of lifetime ofa typical electrochemical battery having a Lithium-based chemistry.

FIG. 6 is a plot 163 of charge capacity as a function of lifetime of atypical electrochemical battery having a Lithium-based chemistry.

FIG. 7 is a diagram of a conventional battery tester 10 for generatingan impedance spectrum of an electrochemical battery.

FIG. 8 is a flowchart of a method 200 in accordance with one novelaspect.

FIG. 9 is a diagram of a pre-determined impulse current signal generatedusing the battery pack #1 and the switching power converter inaccordance with method 200.

FIG. 10 is a diagram of a response signal output by battery pack #1 thatis measured by voltage sense circuitry in accordance with method 200.

FIG. 11 is a diagram of a pre-determined equation used by the processorto estimate the SOH of battery pack #1 using the obtained estimation ofan impedance spectrum in accordance with method 200.

FIG. 12 is a waveform diagram of voltage and current along various nodesof powertrain 115 when processor 134 performs method 200.

FIG. 13 is a diagram of the processor analyzing the obtained estimationof an impedance spectrum to detect a fault in battery pack #1.

FIG. 14 is a flowchart of a method 300 in accordance with another novelaspect.

FIG. 15 is a diagram of a pre-determined sinusoidal current signal usingthe battery pack #1 and the switching power converter in accordance withmethod 300.

FIG. 16 is a waveform diagram of voltage and current along various nodesof powertrain 115 when processor 134 performs method 300.

FIG. 17 is a diagram showing the use of high-frequency (1 kHz to 100kHz) current signals to carry out method 300.

FIG. 18 is a flowchart of a method 400 in accordance with yet anothernovel aspect.

FIG. 19 is a diagram showing how the SOH of battery pack #1 is estimatedby determining the internal resistance.

FIG. 20 is a diagram showing how the SOC of battery pack #1 is estimatedby determining the internal resistance.

FIG. 21 is a waveform diagram of voltage and current along various nodesof powertrain 115 when processor 134 carries out method 400.

DETAILED DESCRIPTION

Reference will now be made in detail to some embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 1 is a perspective diagram of a portion of an electric vehicle 100.The electric vehicle 100 is an electric twenty-passenger shuttle busthat utilizes an entirely electric powertrain. Shuttle bus 100 includesfour Energy Storage Systems (ESSs) 101-104 that store electrical energyutilized by the power train for operating shuttle bus 100. In thisexample, each of the ESSs 101-104 is an electrochemical battery packthat stores a maximum of forty kilowatt hours. (The term “battery pack”is used to refer to an electrochemical battery pack.) Shuttle bus 100has a range of approximately one-hundred miles when all of the batterypacks 101-104 are fully charged. However, the range of the shuttle bus100 is dependent on various factors, such as temperature, age of thebattery packs, and the rate at which energy is discharged from thebattery packs, among other factors.

The battery packs 101-104 each comprises a plurality of electrochemicalstorage cells (referred to as “cells”) and a Battery Management System(BMS). Battery pack 101 comprises a plurality of cells 105 and BMS 106.Battery pack 102 comprises a plurality of cells 107 and BMS 108. Batterypack 103 comprises a plurality of cells 109 and BMS 110. Battery pack104 comprises a plurality of cells 111 and BMS 112. Each of the cells105, 107, 109 and 111 is of a lithium-ion chemistry type. Although eachof the cells depicted in FIG. 1 include cells connected in series, inactuality the cells may be connected in series, in parallel, or incombination of both series and parallel. Various types of batterychemistries and cell configurations exist and the type of chemistry andconfiguration that is selected is largely dependent on the requirementsof the battery pack. Battery packs 101-104 have a capacity of fortykilowatt hours, however, battery packs having a capacity as low as fivekilowatts may be selected depending on the application.

The BMS included in each of the battery packs 101-104 provides amechanism for measuring the voltage, SOC and charge or discharge of thecells. The BMS may provide circuitry for protecting the cells fromcharging or discharging beyond the limits supported by the cells. TheBMS may also include circuitry that performs cell balancing todistribute charge equally among all the cells, or to draw excess powerfrom cells to bring them into balance with each other. Charge balancingtends to prolong battery life because no single cell is overcharged tothe extent that it becomes damaged. Other types of BMSs include robustfeatures whereas other BMSs include less functionality.

Battery pack manufacturers provide battery packs in a single productthat includes both the plurality of cells and the BMS, such as batterypacks 101-104. In the example of FIG. 1, the powertrain of shuttle bus100 supports conventional and widely available battery packs to storeenergy, rather than requiring specially-designed battery packs that aremore costly. In addition, battery packs may be easily replaced with newbattery packs without requiring significant changes to the powertrain.The low-cost integration of conventional battery packs with thepowertrain of shuttle bus 100 is shown by dotted line and arrow 113. Inone embodiment, each of battery packs 101-104 is a CleanBat availablefrom Dow Kokam, 2700 S. Saginaw Road, Midland, Mich. 48640. In anotherembodiment, each of battery packs 101-104 is a Core Pack available fromA123 Systems, LLC.

FIG. 2 is a simplified conceptual diagram of the electric powertrain 115of the shuttle bus 100. Powertrain 115 includes the battery packs 101,102, 103 and 104, battery controllers 116, 117, 118 and 119, a PowerControl Unit (PCU) 120, a three-phase electric motor 121, a motorcontroller 122, an electrical charging device 123, a Direct Current (DC)electrical power bus 124, and a communication bus 125. Batterycontrollers 116-119 communicate with the BMSs (not shown) within thebattery packs and also control current flow between the battery packsand the DC power bus 124. Alternating Current (AC) source 126 provideselectrical power to charger 123, and charger 123 in turn converts the ACpower into DC power and supplies the DC power onto DC power bus 124 forcharging the battery packs 101-104.

Components of powertrain 115 communicate with other components viacommunication bus 125. PCU 120 controls overall system operation of thepowertrain 115 via communication bus 125. The PCU 120 also controls themanner in which each of battery packs 101-104 supplies power to andreceives power from the DC power bus 124. The PCU 120 communicates withmotor controller 122 to control operation of motor 121. Componentscomply with the Controller Area Network (CAN) protocol standard forcommunicating across bus 125. In other examples, the FlexRay protocolstandard is used to communicate across bus 125.

Components of powertrain 115 transfer charge between each other via DCpower bus 124. The PCU 120 is operable to control the batterycontrollers 116-119 such that charge is transferred between the batterypacks 101-104. The PCU 120 is also operable to control the motorcontroller 122 to supply power from battery packs 101-104 to motor 121via the DC power bus 124.

FIG. 3 is a more detailed diagram of battery pack 101, batterycontroller 116 and PCU 120. PCU 120 comprises processor 127, memory 128and local bus 129. Memory 128 is a processor-readable medium that isreadable by the processor 127. Battery controller 116 comprises aswitching power converter 130, a current sense circuit 131, a voltagesense circuit 132, interface circuit 133, processor 134, memory 135, andlocal bus 136. Memory 135 is a processor-readable medium that isreadable by the processor 134. The battery pack 101 comprises theplurality of cells 105 and the BMS 106.

In this example, the switching power converter 130 is a bi-directionalbuck-boost power converter. The bi-directional buck-boost powerconverter 130 comprises N-channel Metal Oxide Semiconductor Field-EffectTransistors (MOSFETs) 137, 138, 139 and 140, diodes 141, 142, 143 and144, capacitors 145 and 146, and inductor 147. Transistors 137, 138, 139and 140 need not be MOSFET devices and may be realized as Insulated-GateBipolar Transistors (IGBTs). The processor 134 controls thebi-directional buck-boost power converter 130 by supplying digital logiccontrol signals SW1, SW2, SW3 and SW4 to converter 130 via conductors148. Digital logic control signal SW1 is supplied to a gate oftransistor 137. Digital logic control signal SW2 is supplied to a gateof transistor 138. Digital logic control signal SW3 is supplied to agate of transistor 139. Digital logic control signal SW4 is supplied toa gate of transistor 140.

In operation, processor 134 controls digital logic levels of the digitalcontrol signals SW1-SW4 such that battery pack 101 is charged ordischarged. If battery pack 101 is to be charged, processor 134 of thebattery controller 116 controls digital logic controls signals SW1, SW2,SW3 and SW4 to appropriate digital logic levels causing current to flowfrom DC power bus 124 through conductor 149 through bi-directionalbuck-boost power converter 130, through current sense circuit 131,through conductor 150 and onto battery pack terminal 151. If, on theother hand, battery pack 101 is to be discharged, processor 134 of thebattery controller 116 controls digital logic controls signals SW1, SW2,SW3 and SW4 to appropriate digital logic levels causing current to flowfrom battery pack terminal 152, through conductor 153, throughbi-directional buck-boost power converter 130, through conductor 154 andonto DC power bus 124. The digital logic levels and timing of theswitching of the controls signals SW1, SW2, SW3 and SW4 depend uponwhether current is flowing from a higher potential to a lower potentialin which case the power converter 130 operates in buck mode, or whethercurrent is flowing from a lower potential to a higher potential in whichcase power the converter 130 operates in boost mode. For additionalinformation on the structure and operation of powertrain 115 and how PCUcontrols and communicates with the battery controllers 116-119 and motorcontroller 122, see: 1) U.S. Patent Publication No. 2011/0089760,entitled “System And Method For Managing A Power System With MultiplePower Components”, filed Oct. 20, 2010, by Castelaz et al., and 2) U.S.Patent Publication No. 2010/0237830, entitled “System and Method forBalancing Charge Within a Battery Pack”, filed Mar. 23, 2010, byCastelaz et al. (the subject matter of each of these patent documents isincorporated herein in its entirety).

FIG. 4 is a diagram showing how an impedance spectrum for anelectrochemical battery shifts over a batteries lifetime. Referencenumeral 155 identifies a first impedance spectrum of a typicalelectrochemical battery having a Lithium-based chemistry. Referencenumeral 156 identifies a second impedance spectrum of the sameelectrochemical battery after aging. The term “aging” refers to theelectrochemical battery undergoing charge and discharge cycles,typically on the order of hundreds of cycles. Reference numeral 157identifies the real-axis that represents the resistance (or realcomponent of the impedance), and reference numeral 158 identifies theimaginary-axis that represents the reactance (or imaginary component ofthe impedance). The impedance spectrums 155 and 156 are represented asNyquist plots. Each point on the Nyquist plot corresponds to afrequency. At higher frequencies, the variance in resistance over thebattery's lifetime is more pronounced than at lower frequencies. Thisphenomenon is identified by reference numeral 159.

FIG. 5 is a plot 160 of internal resistance as a function of lifetime ofa typical electrochemical battery having a Lithium-based chemistry.Reference numeral 161 identifies the independent axis representinglifetime (charge/discharge cycles), and reference numeral 162 identifiesthe dependent axis representing internal resistance (ohms). As indicatedby plot 160, the internal resistance of the electrochemical batteryincreases over the battery's lifetime.

FIG. 6 is a plot 163 of charge capacity as a function of lifetime of atypical electrochemical battery having a Lithium-based chemistry.Reference numeral 164 identifies the independent axis representinglifetime (charge/discharge cycles), and reference numeral 165 identifiesthe dependent axis representing charge capacity (ampere-hours). Asindicated by plot 163, the internal resistance of the electrochemicalbattery increases over the battery's lifetime.

FIG. 7 is a diagram of a conventional battery tester 10 for generatingan impedance spectrum of an electrochemical battery. Battery tester 10utilizes a signal generator 12 to generate and supply an excitationsignal 13 to the battery 20. Battery tester 10 measures the voltagebetween terminals of battery 20 and the current passing into battery 20.The data resulting from these measurements is then used to determine animpedance of the battery 20. Several disadvantages exist with batterytester 10 and other conventional techniques for determining impedance.One disadvantage is that battery tester 10 requires circuitry of signalgenerator 12 to generate the excitation signal. This increases the costand complexity of determining the impedance of a battery in a system.Moreover, such conventional systems are ideal for testing single cells,but ineffective for testing battery packs containing many cells becauseparasitic losses in battery pack connections introduce too much noiseand attenuation for the small excitation signals that are used. Foradditional information on the structure and operation of devicesconventionally used to obtain an impedance spectrum for electrochemicalbatteries, see: 1) U.S. Pat. No. 6,778,913, entitled “Multiple ModelSystems And Methods For Testing Electrochemical Systems”, filed on Apr.29, 2002, by Tinnemeyer; and 2) U.S. Patent Publication No.2012/0078552, entitled “In-Situ Battery Diagnosis Method UsingElectrochemical Impedance Spectroscopy”, filed Sep. 22, 2011, by Mingantet al. (the subject matter of each of these patent documents isincorporated herein in its entirety).

FIG. 8 is a flowchart of a method 200 in accordance with one novelaspect. In one embodiment, battery controller 116 is realized as asingle integrated circuit that performs method 200 to obtain acharacteristic of one of the battery packs 101-104 of the powertrain 115in FIG. 2. In the example of FIG. 8, the characteristic to be obtainedis an estimation of the State Of Health (SOH) of the battery pack. TheSOH of the battery pack indicates the state of degradation of thebattery pack. During the lifetime of the battery pack, the battery packundergoes charge and discharge cycles. Such cycles cause the batterypack to degrade because of the physical and chemical variations thatoccur while the battery pack is operating. The SOH is usuallyrepresented as a percentage corresponding to: the battery pack's totalcapacity at a given time, the number of charge and discharge cyclesremaining, or the likelihood the battery pack will fail within a giventime. FIGS. 9-12 illustrate steps of method 200, as explained below.

In a first step (step 201), electrochemical battery packs of a systemare controlled to supply power onto an electrical power bus and toreceive power from the electrical power bus. In FIG. 3, the memory 128comprises a set of processor-executable instructions 206 that whenexecuted by the processor 127 causes the battery packs 101-104 to becharged and discharged according to the requirements of the powertrain115. For example, if battery pack 101 is to receive charge from the DCpower bus 124, then processor 127 of PCU 120 transmits a CANcommunication 207 to processor 134 of battery controller 116 via localbus 129. Processor 134 in turn controls switching of control signalSW1-SW4 such that the bi-directional buck-boost power converter 130causes charge to be transferred from DC power bus 124 to battery pack101. Each of battery packs 102-104 is controlled in a similar fashiondepending on how processor 127 of PCU 120 determines charge should betransferred throughout powertrain 115.

Next, whether to determine a characteristic of one of the battery packsis decided (step 202). If a characteristic of one of the battery packsis not to be determined, then all the battery packs continue to operatein a normal fashion (step 201). If, on the other hand, a characteristicof one of the battery packs is to be determined, then a pre-determinedimpulse current signal is supplied onto the battery pack (step 203). Forexample, in FIG. 9, the set of processor-executable instructions 206when executed by the processor 127, determines that a SOH characteristicis to be obtained for battery pack 101.

The pre-determined impulse current signal is supplied as follows. InFIG. 9, the memory 135 of battery controller 116 comprises a set ofprocessor-executable instructions 208. When the processor 134 executesthe instructions 208, the processor 134 controls digital logic levels ofthe digital control signals SW1-SW4 such that inductor current 209 rampsup. After the inductor current 209 reaches a current threshold or acertain amount of time elapses, the processor 134 switches the digitallogic levels of the control signals SW1-SW4 so that the battery pack 101supplies an impulse current signal 210 to the bi-directional buck-boostpower converter 130 and onto DC power bus 124. The impulse currentsignal 210 is a forty ampere current pulse. The impulse current signal210 is said to be “pre-determined” because the information required togenerate the current pulse 210 (such as the amplitude of the pulse) isstored in memory 135. In another example, rather than causing impulsecurrent signal 210 to be discharged from the battery pack 101 to the DCpower bus 124, the processor 134 controls digital logic levels of thedigital control signals SW1-SW4 so that an impulse current signal issupplied from the DC power bus 124 through converter 130 and ontobattery pack 101. The impulse current signal is but one type ofpre-determined signal that can be generated and other types ofpre-determined signals may be generated using the battery pack and theswitching power supply.

Next, a voltage between terminals 151 and 152 of the battery is measured(step 204). The voltage is a response signal output by the battery pack101. For example, in FIG. 10, voltage sense circuit 132 detects thevoltage between terminals 151 and 152. The terminal 151 is coupled toBATT+ terminal of cells 105, and the terminal 152 is coupled to BATT−terminal of cells 105. Reference numeral 211 identifies a voltage signaldetected by the voltage sense circuit 132. Voltage signal 211 is atime-domain impulse response of the battery pack 101 resulting fromimpulse current signal 210 input to the battery pack 101. The voltagelevel and time information of voltage signal 211 is stored in memory 135of battery controller 116. In another embodiment, the voltage signalinformation is not detected by voltage sense circuit 132. Instead thevoltage information is obtained by BMS 106 and communicated to processor134 through interface 133 and local bus 136. The voltage sensing of theresponse signal is performed entirely within the battery pack and noseparate voltage sense circuitry external to the battery pack isrequired to obtain response signal information.

Next, the measured response signal is processed to obtain an estimate ofan impedance spectrum for the battery pack (step 205). For example, theprocessor 134 performs signal processing on the information stored inmemory 135 corresponding to the pre-determined impulse current signal210 and the time-domain voltage signal 211. In one embodiment, theprocessor 134 performs a Fourier transform operation, such as a DiscreteFast Fourier Transform (DFFT), on the current and voltage information toobtain an estimate of an impedance spectrum. In another embodiment, theprocessor 134 performs a Spectral Density Estimation on the current andvoltage information to obtain an estimate of an impedance spectrum. Inyet another embodiment, the processor 134 first performs a DFFT. If theprocessor 134 determines that the output spectrum is dominated by noise,then the processor 134 then performs a PSD operation. The result of theprocessing is an estimate of an impedance spectrum of the battery pack101 that includes resistance (real component of impedance) and reactance(imaginary component of impedance) across various frequencies, such asplot 155 of FIG. 4.

Next, a weighted average is calculated of the resistance and reactanceof the battery pack over a plurality of frequencies (step 206). Theresistance and reactance information is obtained from the estimate ofthe impedance spectrum of the battery pack. The result of the weightedaverage calculation is a value indicative of the SOH of the batterypack. For example, in FIG. 11, the processor 134 uses pre-determinedequation 212 stored in memory 135 to process the current and voltageinformation. The equation 212 includes 1×N weight vector 213, N×1magnitude vector 214, 1×N weight vector 215 and N×1 phase vector 216.Weight vectors 213 and 215 comprise real number values that areempirically determined. The selected weights vary across types ofbattery packs, the size of the battery packs, and the chemistry of theelectrochemical cells contained within the battery pack, among otherfactors. Magnitude vector 214 comprises real number values representingthe magnitude of the impedance (the resistance) at an N^(th) frequency.Phase vector 216 comprises real number values representing the phase ofthe impedance (the reactance) at an N^(th) frequency. The result ofprocessor 134 calculating equation 212 is the SOH represented as apercentage.

FIG. 12 is a waveform diagram of voltage and current along various nodesof powertrain 115 when processor 134 performs method 200. Time period T1identifies a period of time before processor 134 carries out method 200to determine the SOH of battery pack #1. Time period T2 identifies theperiod of time in which the SOH is determined for battery pack #1. Atthe transition between T1-T2, the processor 127 of PCU 120 determinesthat the battery pack #1 should output zero current (prior to generatingthe impulse current signal). Reference numeral 218 identifies the dropin current of battery pack #1. To compensate for this drop in current(from approximately twenty-five amperes to zero amperes), the processor127 of PCU 120 determines that the other battery packs #2-#4 shouldtogether increase by the equivalent amount of current (approximatelytwenty-five amperes) to compensate for the drop in current. Referencenumerals 219-221 identify the steep increase in current output bybattery packs #2-#4. As a result of this compensation, any electricalload sourced by the DC power bus 124 will not experience any change inavailable current. For example, if motor 121 were driving shuttle bus100, then motor 121 would not experience any change in availablesupplied current and motor 121 would maintain functionality during thetime interval T2. As such, the characteristic of battery pack #1 isdetermined without perturbing the current supplied to an electrical loadsuch that the current supplied during T2 follows the full-functionalitytime-varying current draw of the electrical load with the same degree ofaccuracy this current draw was followed before the T1/T2 transition anddoes not affect the ability of the load to draw current or perform work.Time period T3 identifies a period of time in which battery pack #1returns to normal operation after the SOH for battery pack #1 isdetermined.

FIG. 13 is a diagram of the processor 134 analyzing the obtainedestimate of the impedance spectrum to detect a fault in battery pack #1.After the estimate of the impedance spectrum is obtained in step 205,processor 134 may then analyze the estimate of the impedance spectrum todetermine whether a fault is present within battery pack #1. Theprocessor determines that the region of the impedance spectrumestimation between frequencies F1 and F2 has a sharp spike indicatingthat at least one of the plurality of cells 105 have degraded. As aresult of detecting the degraded battery pack cell, processor 134transmits a communication to processor 127 of PCU 120 indicating thefault. Processor 127 of PCU 120 transmits a communication over a network(not shown) to prompt operator of shuttle bus 100 that the battery pack#1 is degraded. In one example, the communication is a wireless RadioFrequency (RF) communication that is transmitted by a RF transmitterwithin the PCU 120 and is received on a RF receiver of a notificationdevice located at a shuttle bus station. The notification devicepresents the degraded battery pack information, including identificationinformation for the particular shuttle bus and battery pack, on adisplay so that the shuttle bus station knows to replace the batterypack if the degradation exceeds acceptable levels.

FIG. 14 is a flowchart of a method 300 in accordance with another novelaspect. In one embodiment, battery controller 116 is realized as asingle integrated circuit that performs method 300 to obtain acharacteristic of one of the battery packs 101-104 of the powertrain 115in FIG. 2. Method 300 involves using a plurality of low frequency (0.1Hz to 100 Hz) current signals to obtain response signals in the form ofvoltage signals to then obtain the estimate of the impedance spectrum.Carrying out method 300 yields an SOH estimation of the battery pack.

In a first step (step 301), electrochemical battery packs of a systemare controlled to supply power onto an electrical power bus and toreceive power from the electrical power bus. For example, the PCU 120 ofFIG. 2 controls the charge and discharge of battery packs #1-4 accordingto the power requirements of powertrain 115.

Next, whether to determine a characteristic of one of the battery packsis decided (step 302). If a characteristic of one of the battery packsis not to be determined, then all the battery packs continue to operatein a normal fashion (step 301). If, on the other hand, a characteristicof one of the battery packs is to be determined, then a pre-determinedsinusoidal signal having an initial frequency is set (step 303). Forexample, in FIG. 15, a sinusoidal current signal 310 having a frequencyof 0.1 Hz is set by processor 134 reading amplitude and frequencyinformation from memory 135 via local bus 136.

Next, the predetermined sinusoidal signal is generated using the batterypack and the switching power converter (step 304). For example, in FIG.15 the sinusoidal current signal 310 having a frequency of 0.1 Hz isgenerated using the battery pack 101 and the bi-directional buck-boostconverter 130. The sinusoidal current signal 310 ranges in amplitudefrom −20 amperes to +20 amperes and is generated by the bi-directionalbuck-boost power converter 130 in a technique similar to that describedwith regard to FIG. 8.

Next, a voltage between two terminals of the battery pack is measuredresulting in a voltage signal (step 305). The voltage signal is anoutput response of the battery pack #1. The voltage signal informationand the input current sinusoidal current signal 310 is stored in memory135.

Next, a decision is made whether another sinusoidal current signalhaving a next frequency is to be used to obtain another voltage responsesignal (step 306). If it is determined that another sinusoidal currentsignal is to be generated having a next frequency, then the processor134 reads amplitude and frequency information of the next signal frommemory 135 via local bus 136, and steps 304 and 305 are repeated. In theexample of FIG. 15, a plurality of sinusoidal current signal having afrequency between 0.1 Hz and 100 Hz are used. Typically, less than fivedifferent sinusoidal current signals are used to minimize the timerequired to estimate the SOH of the battery pack.

After the voltage signal information is collected for the plurality ofsinusoidal current signals, signal processing is performed to obtain anestimate of an impedance spectrum of the battery pack (step 308). Next,the estimate of the impedance spectrum information is used to obtain avalue indicative of the SOH (step 309). This is similar to the techniquedescribed with regard to FIG. 11, except equation 212 would includeadditional vector operations to incorporate the additional voltagesignal information in the SOH estimation.

FIG. 16 is a waveform diagram of voltage and current along various nodesof powertrain 115 when processor 134 performs method 300 using apre-determined sinusoidal signal 311 having a frequency of 1 Hz and anamplitude from −20 amperes to +20 amperes. Time period T1 identifies aperiod of time before processor 134 carries out method 300 to determinethe SOH of battery pack #1. Time period T2-T4 identifies the period oftime in which the SOH is determined for battery pack #1. At thetransition between T1-T2, the processor 127 of PCU 120 determines thatthe battery pack #1 should output zero current for a period of time T2(prior to generating the sinusoidal current signal). The processor 127of PCU 120 determines that the other battery packs #2-4 should increaseby the equivalent amount of current to compensate for the drop incurrent.

At the T2/T3 transition, battery pack #1 is controlled to output apre-determined sinusoidal current signal 311. During the T3 time period,the battery pack #1 and the bi-directional buck-boost converter 130generate the sinusoidal current signal 311. Reference numeral 312identifies a measured voltage signal which is output by the battery packin response to generating the sinusoidal current signal 311. The voltagesignal 312 is a time-delayed version of the sinusoidal current signal311. To compensate for the sinusoidal current output by battery pack #1,the processor 127 of PCU 120 determines that the other battery packs#2-4 should output a sinusoidal current to cancel the effect of thesinusoidal current being output by battery pack #1. Reference numerals313-315 identify the current output by the other battery packs #2-4 eachof which is one-hundred and eighty degrees out of phase with thesinusoidal current signal 311 and the sum of the amplitude of eachcancels the effect of signal 311. As a result, the current supplied fromthe battery packs #1-#4 to the DC power bus 124 is unaffected during theSOH determination period (T2-T4). Reference numeral 316 identifies acurrent signal supplied to an electrical load drawing current from DCpower bus 124. As shown, current 316 does not include any sinusoidcomponents during T3. As such, the characteristic of battery pack #1 isdetermined without perturbing the current supplied to an electrical loadsuch that the current supplied during T2 follows the full-functionalitycurrent draw from the electrical load with the same accuracy it didbefore the T2/T3 transition and does not impact the functionality of theelectrical load or the current draw of the electrical load.

During the T4 time period, the battery pack is controlled to output zerocurrent. During the T5 time period, battery pack #1 resumes normaloperation, and battery packs #2 through #4 no longer compensate forbattery pack #1, rather all four battery packs share the load. Thezero-current output during time periods T2 and T4 is not required. Forexample, the battery pack #1 may immediately transition from normaloperation to being supplied with a current signal for SOH determination,and then transition back to normal operation without any zero-currentoperation.

FIG. 17 is a diagram showing the use of high-frequency (1 kHz to 100kHz) current signals to carry out method 300. Reference numeral 317identifies a triangular current signal having a frequency of 1 kHz andan amplitude ranging from +0 amperes to +20 amperes. The triangularcurrent signal 317 is generated in a similar fashion as sinusoidalcurrent signal 310 of FIG. 15.

FIG. 18 is a flowchart of a method 400 in accordance with yet anothernovel aspect. Method 400 provides a method of determining the internalresistance of a battery pack to determine a battery pack characteristic.Carrying out method 400 yields an SOH estimation or an SOC estimation ofthe battery pack.

In a first step (step 401), electrochemical battery packs of a systemare controlled to supply power onto an electrical power bus and toreceive power from the electrical power bus. For example, the PCU 120 ofFIG. 2 controls the charge and discharge of battery packs #1-4 accordingto the power requirements of powertrain 115.

Next, whether to determine a characteristic of one of the battery packsis decided (step 402). If a characteristic of one of the battery packsis not to be determined, then all the battery packs continue to operatein a normal fashion (step 401). If, on the other hand, a characteristicof one of the battery packs is to be determined, then a pre-determinedconstant current signal is generated using the battery pack and theswitching power converter (step 403). The constant current signal isgenerated by the bi-directional buck-boost converter 130. A value atwhich the current signal is held constant is stored in memory 135.

Next, a voltage between two terminals of the battery pack is measured(step 404). For example, voltage sense circuit 132 measures the voltagesignal generated by the battery pack #1 in response to receiving theconstant current signal. The measured voltages that form the voltagesignal are stored in memory 135.

Next, a difference between the measured voltage and the Open CircuitVoltage (OCV) is calculated to obtain the change in voltage (step 405).The processor 134 reads the measured voltages that form the voltagesignal from memory 135 over local bus 136. Next, the internal resistanceis calculated by dividing the change in voltage by the set current (step406). The processor 134 reads one of the measured voltages and thecurrent from memory 135 and computes the difference.

Next, a value indicative of the characteristic is determined by alook-up table operation using the determined internal resistance (step407). The characteristic determined using the internal resistance is SOH(as shown in FIG. 19) or SOC (as shown in FIG. 20) of the battery pack.In the example of FIG. 19, a look-up table 408 that associates aninternal resistance with a percentage indicative of SOH is stored inmemory 135. The processor 134 performs a look-up operation using thedetermined internal resistance to obtain the SOH estimation of thebattery pack. In the example of FIG. 20, a look-up table 409 thatassociates an internal resistance with a percentage indicative of SOC ofthe battery pack is stored in memory 135. The processor 134 performs alook-up operation using the determined internal resistance to obtain theSOC estimation of the battery pack.

FIG. 21 is a waveform diagram of voltage and current along various nodesof powertrain 115 when processor 134 carries out method 400. Referencenumeral 410 identifies the voltage signal output by battery pack #1, andreference numeral 411 identifies the current output by battery pack #1(or the current signal supplied to battery pack #1). During a timeperiod T1, battery pack #1 is made to operate at zero-current level. Thevoltage signal 410 is at the OCV level of 365V. Then, during a timeperiod T2, battery pack #1 is controlled to output a constant current of−20 amperes. At the T1-T2 transition, battery packs #2-#4 compensate forthe sudden increase in current supplied to battery pack #1. Referencenumerals 412-414 identify the steep increase in current output bybattery packs #2-#4 to compensate for battery pack #1. Accordingly, thecharacteristic of battery pack #1 is determined without perturbing thecurrent supplied to an electrical load such that the current suppliedduring T1 continues to match the current draw of the electrical load anddoes not impact the functionality of the electrical load or the abilityof the electrical load to draw current.

Although certain specific embodiments are described above forinstructional purposes, the teachings of this patent document havegeneral applicability and are not limited to the specific embodimentsdescribed above. For example, in another embodiment, the ESS is anelectrochemical battery pack, and the plurality of electrochemicalbattery packs is part of an electrical power grid that includes athree-phase Alternating Current (AC) power bus. Each of the plurality ofelectrochemical battery packs are coupled to the three-phase AlternatingCurrent (AC) power bus through a bi-directional AC/DC converter. Acontroller operates similar to the methods described above to determinethe SOH characteristic and the SOC characteristic of any one of thebattery packs. During estimation of a SOH or SOC characteristic of thebattery pack, if a pre-determined signal is to be supplied to thebattery pack, then the bi-directional AC/DC converter operates in arectifier mode drawing current from the three-phase AC power bus. If, onthe other hand, the pre-determined signal is to be supplied from thebattery pack to the bi-directional AC/DC converter and onto thethree-phase AC power bus, then the bi-directional AC/DC converteroperates in an inverter mode. During the estimation of either the SOH orSOC characteristic, the estimation is performed without disrupting anyelectrical load that sources or sinks current from the three-phase ACpower bus of the power grid. Accordingly, various modifications,adaptations, and combinations of various features of the describedembodiments can be practiced without departing from the scope of theinvention as set forth in the claims.

What is claimed is:
 1. A method comprising: (a) controlling a pluralityof Energy Storage Systems (ESSs) to supply power onto an electricalpower bus and to receive power from the electrical power bus, whereineach of the ESSs is coupled to one of a plurality of switching powerconverters, and wherein each of the switching power converters iscoupled to the electrical power bus; (b) generating a pre-determinedsignal using a first switching power converter, wherein charge flowsfrom a first ESS onto the electrical power bus via the first switchingpower converter, wherein charge flows from the electrical power bus ontoa second ESS via a second switching power converter, wherein current issourced or sinked between the first and second ESSs, and wherein thepre-determined signal is masked from a load during (a) and (b); (c)measuring a response signal output by the first ESS as a result ofgenerating the pre-determined signal in (b); and (d) determining a valueindicative of a characteristic of the first ESS using the pre-determinedsignal and the response signal.
 2. The method of claim 1, wherein thecharacteristic of (d) is State Of Health (SOH), and wherein the SOH is apercentage that indicates an amount of recharge cycles remaining in thefirst ESS until the first ESS is inoperable.
 3. The method of claim 1,wherein the characteristic of (d) is State Of Charge (SOC), and whereinSOC is a percentage that indicates an amount of charge remaining in thefirst ESS.
 4. The method of claim 1, wherein the characteristic of (d)is State Of Health (SOH), wherein the pre-determined signal of (b) is apredetermined current signal and the response signal measured in (c) isa voltage signal, and wherein the determining of the value indicative ofthe SOH of (d) comprises: (d1) processing the response signal to obtainan estimate of an impedance spectrum of the first ESS; and (d2)calculating the value indicative of the SOH of the first ESS usingvalues of the estimate of the impedance spectrum obtained in (d1) and apre-determined equation.
 5. The method of claim 1, wherein thepre-determined signal of (b) is a pre-determined current signal and theresponse signal measured in (c) is a voltage signal, and wherein thedetermining of the value of (d) comprises: (d1) determining a differencebetween a voltage value of the voltage signal measured in (c) and anOpen Circuit Voltage (OCV) of the first ESS, wherein the OCV is apre-determined value; (d2) determining an internal resistance of thefirst ESS by dividing the difference determined in (d1) by a currentvalue of the pre-determined current signal of (b); and (d3) performing alookup operation on a lookup table using the internal resistancedetermined in (d2) to obtain the value indicative of the characteristic,and wherein the characteristic is taken from the group consisting of:State Of Health (SOH) and State Of Charge (SOC).
 6. The method of claim1, wherein the load is supplied power from the electrical power bus as aresult of the controlling of (a), wherein (b) through (d) are performedduring a time interval, and wherein the electrical load continues toreceive power from the power bus and maintain functionality during thetime interval.
 7. The method of claim 6, wherein at least one of theplurality of ESSs is charged during the time interval, and wherein anysubstantial change in the power supplied to the load during the timeinterval is due to a change in power demands of the load and not due tothe generating of the pre-determined signal.
 8. The method of claim 1,wherein each ESS is an electrochemical battery pack that stores at leastfive kilowatt hours, wherein the electrochemical battery packs are partof a powertrain of an electric vehicle, and wherein (b) and (d) areperformed by one and only one integrated circuit.
 9. The method of claim1, wherein the each ESS is an electrochemical battery pack that storesat least five kilowatt hours, wherein the electrochemical battery packsare part of an electrical power grid, and wherein (b) and (d) areperformed by one and only one integrated circuit.
 10. The method ofclaim 1, wherein the electrical power bus is a Direct Current (DC) powerbus, and wherein each of the switching power converters of (b) is abi-directional buck-boost power converter that is coupled between one ofthe ESSs and the DC power bus.
 11. The method of claim 1, wherein thepre-determined signal is selected from the group consisting of: apre-determined constant current signal, a pre-determined impulse currentsignal, a pre-determined sinusoidal current signal, a pre-determinedstep current signal, and a pre-determined triangular current signal, andwherein the response signal measured in (c) is a voltage signal.
 12. Asystem comprising: an electrical power bus that supplies a load; aplurality of Energy Storage Systems (ESSs), wherein each of the ESSs isconfigured to supply energy onto the electrical power bus and to receiveenergy from the electrical power bus; and a plurality of controllers,wherein each of the controllers is coupled to one of the ESSs and isalso coupled to the electrical power bus, wherein each of thecontrollers comprises: a switching power converter, wherein apre-determined signal is generated using the switching power converterby transferring energy from one of the ESSs to the electrical power busvia the switching power converter, wherein energy is transferred fromthe electrical power bus to another of the ESSs, wherein current issourced or sinked between at least two of the ESSs, and wherein powersupplied to the load does not substantially change in response togenerating the pre-determined signal; a sense circuit that measures aresponse signal output by the one of the ESSs as a result of thepre-determined signal that is generated; and a processor that determinesa value indicative of a characteristic of the one of the ESSs using thepre-determined signal and the response signal.
 13. The system of claim12, wherein the characteristic is State Of Health (SOH), and wherein theprocessor determines the value indicative of the SOH by: processing theresponse signal to obtain an estimate of an impedance spectrum, andcalculating the value indicative of the SOH by using values of theestimate of the impedance spectrum and a pre-determined equation. 14.The system of claim 12, wherein the response signal is a voltage signal,wherein the pre-determined signal is a current signal, wherein theprocessor determines the value indicative of the characteristic by:determining a difference between a voltage value of the voltage signaland an Open Circuit Voltage (OCV) of the one of the ESSs, determining aninternal resistance of the one of the ESSs by dividing the difference bya current value of the current signal, and performing a lookup operationon a lookup table using the internal resistance to obtain the valueindicative of the characteristic, and wherein the characteristic istaken from the group consisting of: State Of Health (SOH) and State OfCharge (SOC).
 15. The system of claim 12, wherein the generating of thepre-determined signal, the measuring of the response signal, and thedetermining of the value indicative of the characteristic are allperformed during a time interval, and wherein the load continues toreceive power from the electrical power bus and maintain functionalityduring the time interval.
 16. The system of claim 15, wherein at leastone of the ESSs is charged during the time interval, and wherein anysubstantial change in the power supplied to the load during the timeinterval is due to a change in power demands of the load and not due tothe generating of the pre-determined signal.
 17. The system of claim 12,wherein the pre-determined signal is selected from the group consistingof: a pre-determined constant current signal, a pre-determined impulsecurrent signal, a pre-determined sinusoidal current signal, apre-determined step current signal, and a pre-determined triangularcurrent signal.
 18. The system of claim 12, wherein each of the ESSs isan electrochemical battery pack that stores at least five kilowatthours, and wherein the system is part of a powertrain of an electricvehicle.
 19. A system comprising: a plurality of Energy Storage Systems(ESSs) that supply current to an electrical load, wherein each of theESSs is coupled to each of the other ESSs, wherein charge istransferable between the ESSs; means for determining a value indicativeof a characteristic of one of the ESSs without perturbing currentsupplied to the electrical load, wherein the means: (a) causes apre-determined signal to be generated by operating a power convertersuch that available charge in one ESS increases and available charge inanother ESS decreases without substantially impacting the currentsupplied to the electrical load and such that current is sourced orsinked between at least two of the ESSs, (b) measures a response signaloutput by the one of the ESSs to obtain a measured response signal, (c)processes the measured response signal to obtain an estimate of animpedance spectrum of the one of the ESSs, and (d) determines thecharacteristic using the estimate of the impedance spectrum, and whereinthe characteristic is selected from the group consisting of: a State OfHealth (SOH), and a State Of Charge (SOC).
 20. The system of claim 19,wherein the means is a controller that is coupled to the one of theESSs, and wherein the controller comprises: a switching power converter,wherein the switching power converter and the one of the ESSs are usedto generate the pre-determined current signal; a sense circuit thatmeasures a response signal output by the one of the ESSs; and aprocessor that determines the value indicative of the characteristic ofthe one of the ESSs using the pre-determined current signal and theresponse signal.